Feedback oscillators which take advantage of transistor amplification have been the subject of patents and other publications for more than twenty-five years. However, the application of such oscillators for igniting and operating low pressure gas discharge lamps is of more recent origin. The earlier patents demonstrating that system efficiency is augmented considerably when the operating frequency is above 60 Hertz goes back approximately twenty years.
The recognition of the resulting improvement in system efficiency has resulted in the development of various circuits employing feedback oscillators with transistors and one or more transformers with two or three windings in the feedback circuit, but from the most simple to the most sophisticated circuits, each presenting a variety of protective devices, none have given serious consideration to the increase in operating frequency which results from a decrease in system capacitance on tube burn-out for the automatic control of system current.
The failure to consider frequency increase has relegated present oscillator systems to be utilized only with lamps of small power handling capacities (below 20 watts) with circuit input voltages below 100 volts, and such circuits involve relatively high production costs.
Moreover, at the present state of the art, none of these oscillators benefit from the advantages offered by a double stage of amplification, which reduces to a minimum the current which flows through circuit resistors which, in turn, produces a substantial reduction in heat losses (I.sup.2 R losses). These losses have resulted in relatively heavy losses in other systems. Furthermore, when voltages are in excess of about 100 volts, these oscillators when operated as ballasts lack reliability since, in most cases, under open-load circuit or lamp burn-out conditions, the power transistor is automatically condemned to overheating damage or, as a result of the protective circuits designed to protect against current or voltage overload, the ballast oscillator becomes limited in establishing the initial igniting arc, at which time the oscillator currents and voltages can reach unusually high values, as compared with normal operating values, until the system becomes stabilized.
In other cases, where the input voltages are low, (e.g., 12 volts) the transistor need not necessarily burn out under open circuit conditions. However, under high frequency circuit conditions the oscillator is often converted into a true transmitter radiating signals at radio frequencies which interfere with nearby communication systems.
As a result of these malfunctions in the oscillator driven systems, the conventional 60 Hertz electrical ballast system continues, after some 20 years of experimentation with other forms of circuits, to dominate almost 100% of the fluorescent tube ballast market, despite the many important realizable advantages promised by the more sophisticated solid state ballast systems.